BHUSHAN et al.:

BHUSHAN et al.: 130-GSa/s PHOTONIC ANALOG-TO-DIGITAL CONVERTER WITH TS PREPROCESSOR 685Fig. 2. The TSADC system. The electronic digitizer is an Agilent digitalreal-time oscilloscope. MLL: Mode-locked laser. SC: Supercontinuum module,SMF1 and SMF2: spools of SMF28 fiber. PC: Polarization controller. MOD:Modulator. OC: Optical circulator. FM : Faraday mirror. PD: Photodetector.RF: rf source. EDFAs (A1 and A2).Fig. 1(b) with dotted line. The filtered spectrum is centered at1533 nm and it has 1-dB bandwidth of 17 nm. There is 1 mWof average power in this portion of the supercontinuum.The setup for the 130-Gsa/s TS system is shown in Fig. 2.We start with a nearly transform-limited 17-nm slice filteredfrom the supercontinuum source. This slice is dispersed througha single-mode fiber (SMF-28) of length km. This firstdispersion provides a time aperture of approximately 0.8 ns atthe modulator. The chirped optical pulse is amplitude modulatedwith an 18-GHz tone in a Mach–Zehnder (MZ) modulator. Themodulation depth is 25%. The LiNbO3 modulator limit and thebandwidth of the digitizer set the maximum achievable RF frequencyof our system. Dispersing the waveform a second timethrough a lengthkm stretches the modulation envelopein time. The second dispersion stage is implemented usingan length of SMF28 along with a circulator and a Faradaymirror as shown in Fig. 2. The stretch factor ( ) is given by theratio , [4]. Here, andare the total dispersion in the first and second fiber, respectively.The effective sampling rate for the TSADC is, therefore,129.6 GSa/s. The stretched envelope at 1.11 GHz is detected anddigitized by a single 8-GSa/s 1.5-GHz bandwidth channel of theAgilent 54 845A Infinium Oscilloscope. The digitizer has eightnominal bits of resolution. Attempt was made to match the fullscale of the digitizer with the peak-to-peak signal. Exact matchcould not be assured however because the slight variation of thesupercontinuum intensity over the filtered range [see Fig. 1(b)]increases the signal dynamic range beyond the peak-to-peakvoltage swing. The post-stretched time aperture for the systemis 13 ns providing a duty cycle of ns ns . Thedispersion penalty in the system is given by ,where is given by the following [10].Here, is the modulation frequency and is the groupvelocity dispersion parameter. In this system, the dispersionpenalty is 0.55 dB.The digitized output (symbols) is shown in Fig. 3 along withan ideal sine curve (solid line). The bandwidth is limited to 1GHz (prestretch) around 18 GHz using a digital filter. Basedon the standard sine-fit method, [11], the signal-to-noise ratio(SNR) is 45.2 dB, which translates to 7.2 effective number ofFig. 3. The RF signal captured by the TSADC. The points are digitizedsamples and the solid line is the theoretical sine fit.Fig. 4. Error between the sampled data and the ideal sine curve. One LSB isequal to the peak-to-peak voltage divided by 128 (2 ).bits (). Performing this test over141 optical pulses yields a resolution of 7.16 0.6 bits.Fig. 4 shows the the error between the sampled signal and theideal sine curve plotted in the range to . OneLSB is defined as the peak-to-peak voltage divided by 128 (2 ).The maximum error is approximately 0.375 LSB, suggestingthat the TSADC has a SNR corresponding to slightly better thanseven ENOB of resolution.We note that this system is operating in a suboctave rangeand the single tone measurements are not indicative of systemlinearity. The primary source of harmonic distortion in the TSpreprocessor is the sinusoidal transfer function of the MZ modulator.When biased at the quadrature point, the primary sourceof nonlinearity is the third-order harmonic. If is the averageoptical intensity at the output of the quadrature biased modulatorandis the RF signal at the output, then we canshow that the SFDR is given by , [12]. For this system, which translates to an SFDR of 51.6 dB.To understand the fundamental performance limit of theTSADC, we model the system as an amplified optical linkshown in Fig. 5. We note that invoking this model impliesthat the CNR remains the same at the input and output of thesupercontinuum fiber. In other words, the supercontinuum fiberis modeled as passive element with negligible attenuation. InFig. 5, denotes the loss, denotes the amplifier gain, and